1. Field
Embodiments of the invention generally relate to communication systems. More particularly, the invention relates to power control and random backoff control for access probes in communication systems.
2. Background
Wireless communication systems have developed through various generations, including a first-generation analog wireless phone service (1G), a second-generation (2G) digital wireless phone service (including interim 2.5G and 2.75G networks) and a third-generation (3G) high speed data/Internet-capable wireless service. There are presently many different types of wireless communication systems in use, including Cellular and Personal Communications Service (PCS) systems. Examples of known cellular systems include the cellular Analog Advanced Mobile Phone System (AMPS), and digital cellular systems based on Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), the Global System for Mobile access (GSM) variation of TDMA, and newer hybrid digital communication systems using both TDMA and CDMA technologies.
The method for providing CDMA mobile communications was standardized in the United States by the Telecommunications Industry Association/Electronic Industries Association in TIA/EIA/IS-95-A entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” referred to herein as IS-95. Combined AMPS & CDMA systems are described in TIA/EIA Standard IS-98. Other communications systems are described in the IMT-2000/UM, or International Mobile Telecommunications System 2000/Universal Mobile Telecommunications System, standards covering what are referred to as wideband CDMA (WCDMA), CDMA2000 (such as CDMA2000 1xRTT, “1x”, and 1xEV-DO standards, “1xEV”, for example) or TD-SCDMA.
In wireless communication systems mobile stations or access terminals receive signals from fixed position base stations (also referred to as cell sites or cells) that support communication links or service within particular geographic regions adjacent to or surrounding the base stations. In order to aid in providing coverage, each cell is often sub-divided into multiple sectors, each corresponding to a smaller service area or geographic region. An array or series of base stations placed adjacent to each other form a communication system capable of servicing a number of system users, over a larger region.
Conventionally, each mobile station monitors a control channel that can be used to exchange messages between the mobile station and the base station. The control channel is used to transmit system/overhead messages, whereas traffic channels are conventionally used for substantive communication (e.g., voice and data) to and from the mobile station. For example, the control channel can be used to establish traffic channels, control power levels, and the like, as is known in the art. Generally, there are two types of power control for the reverse link, open-loop and closed-loop power control. The open-loop power control conventionally occurs prior to the mobile terminal establishing contact with a base station. The closed-loop control occurs after the mobile and the base station are in communication and the base station can measure the received power levels and feedback power level adjustments to the mobile terminal.
In the open loop condition, the reverse link power for an initial communication signal (e.g., access probe) from the mobile terminal to the base station can be determined by monitoring specialized signals from a base station or access point. For example, in CDMA systems a pilot signal can be use to estimate the channel condition and then determine a power estimate for transmitting back to the base station. The accuracy of the channel conditions and power estimation can greatly impact performance of the system, particularly in terms of latency of the system. For example, 1x and 1xEV systems will transmit an access probe at a first power level based on a power control algorithm. If the first access attempt does not succeed, then the probe is resent at increasingly higher power levels, until it is successful or the power level maximum is reached.
In addition to the message loss due to power related issues (e.g., channel fading, time-varying ROT, etc.), access channel losses can also occur because of access probe collisions, which may be the case in geographically dense group calls. Losses that are caused due to the fading of the wireless channel can be minimized by increasing the transmit power of access probes. Losses due to probe collisions over the Access channel can be mitigated by ensuring that probe transmissions that are synchronized with respect to each other do not transmit their probes at the same time.
Accordingly, a method and system for jointly determining the transmit power of the access probe and the random backoff interval over which the access probe can defer its transmission can improve the system performance by reducing delays due to unsuccessful access probe transmission for geographically dense calls in wireless communication systems such as CDMA2000 1X-A and 1xEVDO networks.